This invention relates to electrical energy meters and to a current probe for use in such meters.
Standard electro-mechanical electrical energy meters have some or all of the following disadvantages.
They all consume a significant amount of power to operate. The IEC standard for class II meters is  less than 2 watts. This power consumption amounts to between 0.25% to 0.5% of all power consumed. Losses due to metering are therefore substantial.
They have inertia problems when starting; therefore they must have a certain amount of power being drawn before they start to register.
They can only be installed by skilled personnel, and their installation is time-consuming. Electro-mechanical meters need to be fixed firmly to a flat surface in an upright position. In territories such as the former Soviet Union when metering is being installed in volume for the first time, the cost of installation of the electro-mechanical meters is high.
In conventional one wire current probes (see FIG. 1), a loop 1 of magnetic material surrounds a current carrying conductor 2 and a coil 3 comprising a large number of turns of wire is wound on the magnetic material 1. This type of probe relies on Ampere""s Law which states that the integral of the magnetic field around a closed loop surrounding a current source is equal to the current enclosed.
In a well designed probe of this kind the voltage or current induced in the coil 3 is not dependent on the position of the source current (conductor 2) within the cross section surrounded by the closed magnetic core 1. Furthermore, the ratio of pickup voltage or current from the current source 2 within the closed magnetic ring core 1, compared to the pickup from the same source when it is located outside the closed magnetic ring core is very large, e.g.  greater than 1000:1.
This ensures that stray pickup from interfering current sources which may be located close to the probe but outside the magnetic ring core do not affect the measurements obtained from the required source which is located inside.
One of the disadvantages of this type of probe however is its cost. The magnetic core must be manufactured in two or more sections to allow the core to be opened and closed so that the conductor can be inserted. In order to make an accurate measurement the alignment of the two sections on closing is critical, as is the requirement that even a small air gap between sections on meeting is not allowed.
U.S. Pat. No. 5,057,769 discloses a probe having a gap 4 (see FIG. 2) in a continuously wound non-magnetic core coil 5 to allow the insertion of the current source. In order to maintain the desirable features of the continuous winding closed non-magnetic core 5, an effort is made to add back in the voltage component that would have been picked up by the coil turns which were removed to provide the air gap 4, by adding two individual multi-turn coils 6 at either side of the gap 4.
Even with the correct number of turns in these coils this is only partly successful. The voltage pickup of the probe is dependent on the location of the source conductor within the internal cross section of the coil. The closer the source current carrying conductor is to the gap or the windings, and the larger the gap, the greater the variation in pickup.
Furthermore, with this design, the pickup from sources in area 7 outside the core gap cross-section is no longer negligible and the pickup from an external current source increases as the gap increases, or as the external sources approach the gap. This can pose a serious limitation especially when measurements are being performed in a distribution box for example, where there may be a large number of conductors carrying various currents in a confined space.
It is an object of the invention to provide a low cost, low power meter which is quick and easy to install and which may, if desired, be retro-fitted to existing mains installations. In particular, it is an object to provide a meter which may be fitted easily to domestic power supplies.
It is a further object to provide an improved probe exhibiting less interference from external sources than in the prior art, without resorting to expensive designs.
According to the present invention there is provided an electrical energy meter comprising an electrically insulating housing for securing relative to least two mains cables each having a conductive core surrounded by a sheath of insulating material, the housing including respective electrical contact means for piercing the insulating sheath of each cable to make contact with the core, sensing means for providing an output corresponding to the current flowing in at least one of the cables, and circuit means for calculating and displaying electrical energy as a function of the voltage across the contact means and the output of the sensing means.
In a further aspect, the invention provides a current probe for measuring current in a conductor, comprising a plurality of coils connected together in series in an arrangement which substantially surrounds the cable in which current is to be measured.
Preferably, said coils are substantially equidistantly spaced in the form of an open loop, with a gap being provided between two of the coils in the loop, said gap enabling introduction of the conductor into the interior of the loop.
In a particularly preferred current probe, the coils are arranged in two concentric loops of coils, each loop being connected in series, and each loop having a gap between two of the coils in the loop, said gaps enabling introduction of the conductor into the interior of the concentric loops.
Preferably, in such an embodiment, there is also provided an electronic circuit for comparing the pickup from external sources experienced by each of the two loops and providing an output which compensates for such pickup, based on the respective dimensions of the loops.